A soundproof wall that reduces sound passing through a structural member has conventionally been known. Soundproof walls are installed at roads, railroads, or factories to prevent noise generated from noise sources such as automobiles from leaking to the surroundings. In addition, a sound or vibration reduction apparatus is also known which reduces sound generated from structural members of various devices such as air-conditioners, printers, and copiers, or the vibration of the structural members.
Passive sound control that reduces sound with each portion of a structural member without the need of the input of energy such as electric power is used for a typical soundproof wall, or sound or vibration reduction apparatus. A gypsum board that reflects sound, glass wool that absorbs sound, and the like are incorporated in, for example, a structural member of the soundproof wall to reduce sound with each portion of the structural member.
In recent years, active sound control that reduces noise actively with the input of energy such as electric power has been proposed to further reduce sound (see Patent Literature 1). The active sound control includes a microphone, a speaker, and an electronic circuit that generates cancelling sound from the speaker. The principle of noise cancelling is to generate, from the speaker, a sound wave component of an opposite phase to noise picked up by the microphone, cancel out each other, and reduce the noise.